i knew i had something concerning (to me) to ask from the last few days:

inspected a delonghi 3kW oil filled rad (with digital this that and the other controls) and found the fuse carrier in the plug had melted (13A fuse still continuous though) and strong smell of melted plastic etc - radiator still operating.

before switched off, it was plugged into a socket on a ring final and the socket face was scorched brown, bubbled - though no signs of scorching internally (looking through the shutters)

fitted new socket plate.

suspecting an overcurrent, I cut plug off rad and fitted another plug and plugged it back in and tested current draw - consistent 12.7A (ran it for 5 mins or so) in line with rating on rad.

have to say live pin on new plug got a bit warm again (maybe not melting temp though) to say the least.

tried another socket, same thing it seemed.

tested current draw at cu - 12.7A (9.4 on one leg and 3.3 on the other)
tested continuity etc. all ok

i cant fathom why the plug got hot and melted the original plug and new one seems to get a bit warm too.

Is this thing normal on these appliances; have i missed anything obvious as i cant think what it might be?

Just had a phone call today - similar thing with a dishwasher installed in a kitchen/diner extension I wired 2 years ago.
They report a burn mark on the socket - I'll be checking tomorrow, but it does seem to me that the loads for so-called portable appliances like these do seem to be somewhat high for something which connects via the humble 13A plug.

A 20A DP switch and 2.5 HR flex would be a stouter solution.

-------------------------
Political correctness is a doctrine, fostered by a delusional, illogical minority, and rabidly promoted by an unscrupulous mainstream media, which holds forth the proposition that it is entirely possible to pick up a turd by the clean end.

Good quality plugs and sockets will not cause this problem.
You can replace to your hearts content rubbish with rubbish and the problem will not go away.
Good engineering principles rule, not price (or in todays world, name)!

The house is one of those expensive but quickly thrown up houses which seem to suffer from the 'throw it up quickly' syndrome with one or two consequences in quality.

That aside, I did notice that on some of the the back-boxes I looked at are quite shallow in depth cut into block and the socket faces may be argued to be pressing the incoming cables to some degree.

How likely is that to cause such a significant problem I described?
... and to seemingly only affect the plug (which appeared to have scorched the plate externally as I said) and not the socket plate internally or the t&e ?!

I feel like re-cutting the back-box just to see - but there could be a few like this in this 'modern expensive' house.

On the subject of crap products, what brand of socket face plates would you recommend as 'well engineered' given I though compliance with BS ought to make sure all those with that standard are good enough for the the job... or is it a case of there may be one or two good ones in a batch of the less fancied brands, but more with suspect quality.

My guess would be initially a loose connection - probably in the plug, less likely the socket, or as Spin suggests the contact between the two - probably on L. Together with the heat from the fuse, it can quickly run-away, the heat increasing the resistance of the joint further, which produces more heat, which ...

It's not that uncommon to see scorch marks around the L pin on 13A plugs/sockets.

As Norm says, replacing with good quality components, carefully assembled, will usually solve the problem.

I would have to agree. Many seem to go for cheapness over quality these days, and the results really are as expected. IME the MK range are still about the best quality, albeit not quite as robust as they used to be. They are however made from plastics that mark with heat, rather than melt, as well as being tested in excess of the BS requirements. I still prefer a good quality 13A plug to a moulded type, particularly where loads are high and for a period of time.

I've got a strong feeling that BS 1363 specifies performance requirements for sockets and FCUs (temperature rise for given currents at given ambient temperatures) - I'd be surprised if it didn't include plugs in that too.
- Andy.

It appears that the problem is (almost) always with the live and hence fused pin. This appears to be a flaw with the concept of a fused plug. I have not seen it in lands with unfused plugs.

A fuse is designed to get hot (in fact to reach the melting point of copper) and I suspect that the heat generated tends to weaken the contact springs resulting in more heating. Cheaper plugs will most probably contain less metal to conduct the heat away and the contact springs will be made of lower grade materials.

Originally posted by: rogerbryant. . . A fuse is designed to get hot (in fact to reach the melting point of copper) and I suspect that the heat generated tends to weaken the contact springs resulting in more heating. . .

I also wonder whether it is something to do with the "soft plastic" used for moulded plugs versus the "hard plastic" for the re-wireable type. There also seems to be a lot more "thermal insulation" round the fuses in the moulded type, and less air round the fuse as well.

In my opinion the use of plugs and sockets for continuous loads at or approaching 13A is a no-no. I have had umpteen occasions where catering equipment (Ban Maries) in our canteen kitchen have their plugs burning up - mainly located around the fuse and/or L pin.
The load is a near constant 12.7A. I have replaced plug and socket each time with MK ones.
I think the design of the fuse carrier is poor as it cannot sustain amperages towards 13A.

In a moulded plug, the clip in the fuse holder connected to the L pin is often riveted to the conductor going to the L pin.

Given the soft plastic, if there is a slight problem with the riveting, i.e. not enough pressure, or small change in the use of the rivet material, this can cause slightly loose connection, and eventually overheating.

I've seen this on appliances at around 1.5 kW too.

There have also been problems with the brazing/welding/crimping procedure on some moulded plugs, where strands of the flex are broken off in the process. This can cause localised heating, and may affect L and N (and of course lead to higher E continuity resistance too) - it's Hobson's Choice which one you get!

Both of these can cause problems for single phase induction motors where the start-winding remains in-circuit at all times ("capacitor run"), if the volt-drop to the appliance becomes too high (tumble dryers on extension leads, or in detached garages fed from the "2.5 sq mm spur off the ring-main", are the biggest casualties it would seem).

Note: not advocating the "2.5 sq mm T&E spur off the ring-main" solution, which is often not conformant to hte Wiring Regs